An EU operation reduces uplink (UL) latency, improves throughput, and provides more efficient use of physical radio resources. During EU operation, an H-ARQ process is used to support EU transmissions between a WTRU and a Node-B including the facilitation of a feedback process for reporting successful or unsuccessful EU data transmissions.
A number of EU H-ARQ processes are defined for each WTRU, and each WTRU supports multiple instances of H-ARQ processes simultaneously. Since a feedback cycle for each EU data transmission is relatively long when compared to UL transmission time, and a different number of transmissions may be required to achieve a successful transmission for each EU transmission, a WTRU is required to operate several H-ARQ processes simultaneously to provide increased data rates and reduced latency.
For any WTRU connection, multiple logical channels exist. These logical channels have different throughput, latency, error rates, and quality of service (QoS) requirements. To satisfy these requirements, the RNC sets a priority for each logical channel known as a medium access control (MAC) logical channel priority (MLP). The MLP is mapped to a dedicated channel MAC (MAC-d) flow which is connected to the EU MAC (MAC-e), which manages the EU H-ARQ processes.
A similar design exists for high speed downlink packet access (HSDPA) in a downlink (DL) channel. When higher priority data is required to be transmitted and all H-ARQ processes are already assigned for transmission of lower priority data, it is allowed to preempt the existing H-ARQ transmissions of lower priority with a higher priority transmission. When the preemption occurs, the lower priority data is rescheduled for an H-ARQ transmission at a later time.
A problem with H-ARQ process preemption is a loss of the benefit of combining. One important advantage of an EU H-ARQ operation is the ability to store received data from previous transmissions and to combine the previous transmissions with subsequent transmissions to increase the probability of a successful data transmission. However, when the H-ARQ processes are preempted, the stored data of the previous transmissions, and thus, the combining advantage of the H-ARQ processes is lost.
A reason for implementing H-ARQ process preemption is that the number of H-ARQ processes that can be configured in the WTRU is limited. While each H-ARQ process requires considerable memory for reception processing, the amount of memory in the WTRU is limited.
Because it is common to have a large amount of lower priority data and a small amount of higher priority data, when processing lower priority transmissions, it is necessary to avoid blocking of higher priority transmissions in order to maintain QoS requirements of the higher priority data. If lower priority data monopolizes the H-ARQ processes, it may degrade overall system performance. Moreover, since lower priority data allows greater latency, it can result in greater H-ARQ process holding time.
H-ARQ process preemption may solve the transmission prioritization problem, but at the expense of the loss of the combining benefit and, correspondingly, the less efficient use of radio resources. It is expected that the best overall performance is achieved in H-ARQ systems when a large percentage of the first and possibly second transmissions fail because a less robust modulation and coding scheme (MCS) requiring far less physical resources can be applied. In this case, when H-ARQ process preemption is employed, these initial transmissions and retransmissions will frequently have to be repeated to achieve successful transmission, which wastes radio resources utilized for the initial preempted transmissions.